Documents filed with American regulators show the hardware, due to be introduced in later this year in an experimental form, uses "bone conduction" to create sound instead of a traditional speaker.

The technology, which sends vibrations to the inner ear through the skull, is not new but has not been widely adopted. Panasonic introduced a prototype set of bone conduction headphones at this year's Consumer Electronics show, however.

A major advantage of bone conduction audio is that it allows the listeners to hear the noise in the environment too. For a Google Glass wearer crossing a busy street the technology could be a life saver.

Google filed a patent for bone conduction spectacles last month, and the Federal Communication Commission this week published it approval for Google Glass, including "integral vibrating element that provides audio to the user via contact with the user's head".

As well as unusual audio, the spectacles feature Wifi and Bluetooth connectivity, and a small screen that appears in the wearer's normal field of vision. The tiny computer inside Google Glass runs the Android mobile operating system and responds to voice commands.

It is planned that wearers will be able to summon up maps and other useful data from the web with having to look at a smartphone or other mobile device.

The first complete Google Glass hardware will be sent to developers who have paid $1,500 to help the firm refine the technology. Google has said it hopes to introduce Google Glass commercially in 2014.